Crop producers face increasingly complex decisions in choosing strategies for protecting the value of their crops. Failure to protect the crop from damage caused by pests can have catastrophic consequences, resulting in an economic loss to growers and higher prices for consumers. Reliance on traditional chemical pesticides to protect the crop has a variety of disadvantages including health and environmental problems, and pest resistance.
Society places a very high value on the environment and on the safety of the food supply. In his nationally broadcast speech at the Democratic National Convention Aug. 29, 1996, President Clinton praised recently passed legislation aimed at reducing pesticide residue in the food supply. The presence of this issue in such an important policy speech reflects the widespread desire to reduce pesticide residue. This increased demand for lower pesticide residues and the concomitant desire to reduce nontarget effects of pesticides has resulted in significantly fewer pesticides being under registration for use. Some of the most harmful pesticides have lost registration due to EPA action, some have been voluntarily withdrawn (such as propargite on several crops) and others have been allowed to lapse rather than being re-registered. The net impact of these pressures is that fewer pesticides are now available for legal use on crops.
With only a few legal pesticides available for use on each crop, pest resistance becomes another concern for the growers. Repeated use of a single pesticide, especially against pests with multiple generations in a single growing season, is the outdoor equivalent of a laboratory experiment aimed at developing an insect strain resistant to the pesticide. The same result occurs outdoors as in the laboratory experiment, and resistant insects begin to appear, often increasing their populations very rapidly once resistance to the maximum applied insecticide rate is achieved. In the past, when resistance began to occur, growers simply could turn to pesticides with different active ingredients to restore control of the pest. With fewer pesticides available, and with the appearance of many insects which are now resistant to multiple pesticide types, growers can be forced to watch helplessly as their crops are destroyed. This has been the case recently in the Imperial Valley of California with the multiply resistant sweet potato whitefly.
Integrated Pest Management, or IPM, has been developed as a tool to increase the efficiency of pesticide use. Rather than applying the pesticide according to a recommended schedule, growers now rely on thorough scouting of the crop to determine the identity, location and relative number of pests. Armed with this information, growers can apply pesticides only where they are needed and only if the pest pressure is likely to cause economic harm if left untreated. IPM results in decreased overall pesticide use, which results in lower food residues and fewer non-target effects. The biological benefit of IPM is that decreased pesticide usage and the tolerance of low levels of pests conserves natural enemies such as predators, parasites and parasitoids which can aid in control of pests. These natural enemies often are more sensitive than the pests and present at lower levels than the pests, meaning that they often are totally eliminated from the crop system under a program of indiscriminate spraying.
Economic forces stand in the way of the natural extension of IPM to include the mass rearing and intentional release of natural enemies to provide increased control of pests. Although many beneficial arthropods have been identified and laboratory scale work has indicated promise for their use in such a program, their implementation in IPM has been minimal. Indeed, biological agents in general account only for about 1% of pesticide sales.
The primary factor preventing wider employment of natural enemies in IPM is the cost of the natural enemies. The most successful biological pesticide, Bacillus thuringiensis, is successful because it can be produced at very low cost in large scale bacterial fermenters and subsequently sold at prices competitive with traditional pesticides used to control the same lepidopterous pests. However, the second most successful biological agent, Phytoseiulus persimilis, is sold at about ten to twenty times the cost of chemical pesticides used to control the two spotted spider mite. At this time P. persimilis is used only on very high value crops: strawberries, greenhouse vegetables and nurseries. These high value niche markets have no other effective legal pesticides capable of controlling the two spotted spider mite. Only when the cost of P. persimilis can be reduced to the level of chemical alternatives can it be expected to compete on other, larger crops such as corn, where its efficacy already has been demonstrated.
P. persimilis and other natural enemies besides those produced in fermenters are too expensive for use on most crops because of the high cost of producing them. In most cases, these natural enemies are grown on prey or host insects which must first be reared, often on a host plant. This process is very labor intensive and space intensive. Replacement of the prey or host with an artificial diet and development of associated mass production technology with decreased labor inputs could bring these costs down dramatically.
The principal phytoseiid mites which are commercially available at this time are Phytoseiulus persimilis, Metaseiulus occidentalis and Amblyseius cucumeris. Of these, M. occidentalis and P. persimilis are obligate predators when in their natural habitat, consuming only prey. A. cucumeris, on the other hand, consumes both prey (thrips rather than spider mites in this case) and pollen encountered on plant surfaces. Both M. occidentalis and P. persimilis are very efficient predators of spider mites. P. persimilis currently accounts for most predatory mite sales worldwide.
In the laboratory, neither M. occidentalis nor P. persimilis has been cultured in the absence of prey (see Kennett, C. E. and J. Hamai, (1980) "Oviposition and development in predacious mites fed with artificial and natural diets (Acari: Phytoseiidae)" Ent. Exp. Appl. 28:116-122, for example). Instead, a plant is grown and infested with spider mites. Predator mites are then released to feed on the spider mites (Gilstrap, F. E. (1977) "Table-top production of tetranychid mites (Acarina) and their phytoseiid natural enemies" J. Kan. Entomol. Soc. 50:229-233). This three component system of plant, pest and predator is the basis for commercial production of M. occidentalis and P. persimilis. Separate facilities are usually maintained for each of the three components of the system, adding significantly to production costs.
An additional large cost component for conventionally grown P. persimilis is that these mites must be packaged for transportation and application to the crop in the absence of food, since this food is a pest. Thus, it is not possible to store mites ready for shipping for extended periods. The result of this is that it is standard practice to overproduce P. persimilis by as much as 50% of anticipated demand, so that surges in demand can be met.
The nutritional requirements of mites have been studied. See, for example, J. A. McMurtry and J. G. Rodriguez (Nutritional Ecology of Phytoseiid Mites, Chapter 19, pp 609-644) and J. G. Rodriguez and L. D. Rodriguez (Nutritional Ecology of Phytoseiid Mites, Chapter 5, pp 177-208). Also, various attempts have been made to develop artificial diets for mites and other organisms. See, for example, Reinecke (Nutrition: Artificial Diets, Chapter 9, pp 391-419, In: Comprehensive Insect Physiology Biochemistry and Pharmacology, Kerkut and Gilbert, ed. (1985), Vol. 4) and Singh (Artificial Diets For Insects, Mites, and Spiders, Department of Scientific and Industrial Research, Auckland, New Zealand, pp 1-21). Diets specifically designed for mites are described in McMutry and Scriven (Effects of Artificial Foods on Reproduction and Development of Four Species of Phytoseiid Mites (1966) Annals Entomol. Soc. Amer. 59:267-269); Shehata and Weismann ("Rearing The Predacious Mite Phytoseiulus Persimilis Athias-Henriot On Artificial Diet" (1972) Biologia (Bratislava) 27(8):609-615); Ochieng et al. ("An Artificial Diet For Rearing the Phytoseiid Mite, Amblyseius teke Pritchard and Baker" (1987) Experimental & Applied Acarology 3:169-173); Hanna and Hibbs ("Feeding Phytophagous Mites on Liquid Formulations" (1970) Journal of Economic Entomology 63(5): 1672-1674). Diets for insects such as coleopterans have also been described (Atallah and Newsom (1966) "Ecological and Nutritional Studies on Coleomegilla maculata De Geer (Coleoptera: Coccinellidae). I. The Development of an Artificial Diet and a Laboratory Rearing Technique" Journal of Economic Entomology 59(5):1173-1179; Smith (1965) "Effects of Food on the Longevity, Fecundity, and Development of Adult Coccinellids (Coleoptera: Coccinellidae)" The Canadian Entomologist 97:910-919; and Vanderzant (1969) "An Artificial Diet for Larvae and Adults of Chrysopa carnea, an Insect Predator of Crop Pests" Journal of Economic Entomology 62:256-257). There has been no report of a low cost diet for effectively raising Phytoseiid mites.
An artificial diet with associated low cost mass production technology and equipment for P. persimilis and other phytoseiid mites would revolutionize their production while providing an opportunity to greatly increase use of these predators. Cost-effective release of inundative amounts of phytoseiids has the potential for dramatically reducing use of conventional insecticides without increased crop loss. Lower costs for phytoseiids also should increase the range of crops on which these predators could be components of IPM programs.